1. Field of the Invention
The subject matter of this invention resides in the field of ceramics and finds particular utility in the areas of dentistry, orthopedics, electronics and electrical engineering.
2. Description of the Prior Art
Much current dental research is focused on the preparation of materials which can be used as a substitute for tooth and bone, as a dental restorative material for fillings, caps and crowns and as a prosthetic filling material for bone. Dental research also is directed to preventing the formation of dental plaque, the putative agent of both dental caries and periodontal disease.
Currently used filler materials for dental restorative compositions such as quartz, alumina, silicates, glass beads, etc., bear little chemical or physical resemblance to tooth enamel. A particular deficiency of these materials lies in the incompatibility of the linear coefficients of expansion of filler material and tooth which can eventually result in marginal leakage and new caries formation. The dental profession, therefore, has long desired a dental filling composition with physical properties which closely conform to those of natural tooth structure.
Furthermore, in the field of surgical prosthetic materials, which is currently dominated by high-strength, non-corrosive alloys, there is a recognized need for a material which more closely resembles biological hard tissue as the problems of tissue acceptance and adherence have not as yet been completely resolved [Hulbert, et al., Materials Science Research 5, 417 (1971)].
In research directed to the discovery of effective anti-plaque chemotherapeutic agents there is need for a standard test material having a tooth-like surface with respect to both plaque formation and substantiveness of chemical agents. Although natural teeth have been used for this purpose, these have the drawbacks of being highly variable, relatively unavailable in large numbers, and require elaborate cleaning before use. Consequently there are used other materials upon which dental plaque will accumulate such as powdered hydroxylapatite, acrylic teeth, glass and wire. Although perhaps adequate for studying plaque formation as such, these materials bear little resemblance to the natural tooth surface and are therefore not completely suitable for use in finding effective anti-plaque agents. For example, it is known that chemicals which inhibit plaque formation on teeth do not necessarily do so on glass and wire [Turesky et al., J. Periodontology 43, 263 (1972)]. There is a need then for an inexpensive, readily available material which is chemically similar to tooth enamel, hard, dense, and highly polished.
Hydroxylapatite, Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2, also known as basic calcium orthophosphate, occurs as a mineral in phosphate rock. It constitutes the mineral phase of tooth and bone and has been suggested as suited to the various purposes outlined above.
U.S. Pat. No. 2,508,816, issued May 23, 1950 discloses a method for obtaining the hydroxylapatite of tooth enamel and its use in admixture with a synthetic resin as a prosthetic tooth composition. This procedure is lengthy and laborious and limited to producing finely divided hydroxylapatite. Moreover, the method is of course dependent on the availability of a supply of natural teeth.
The use of porous, non-ceramic hydroxylapatite as a filler material in dental cements and filling compositions has been disclosed, e.g. in U.S. Pat. No. 3,873,327, issued Mar. 25, 1975 on an application filed Feb. 28, 1974 and German Offenlegungsschrift No. 2,415,333, published Oct. 17, 1974.
Kutty [Indian J. Cham. 11, 695 (1973)] has reported the results of a study of the thermal decomposition of hydroxylapatite which indicate in summary that "Hydroxylapatite, Ca.sub.5 (PO.sub.4).sub.3 (OH), undergoes a slow decomposition when heated at 1250.degree. C. in a current of dry air, forming a mixture of Ca.sub.3 (PO.sub.4).sub.2 and Ca.sub.4 P.sub.2 O.sub.9 as confirmed by IR and X-ray diffraction studies."
The report also described the heating of powdered synthetic hydroxylapatite at 1050.degree. C. and 1150.degree. C. for 20 hours. Excepting the statement that the sample heated at 1150.degree. C. underwent partial decomposition as indicated by infrared and X-ray diffraction data, the report neither described nor characterized the products obtained by heating below 1250.degree. C. nor are said products stated or suggested to have any utility.
Bett, et al, [J. Amer. Chem. Soc. 89, 5535 (1967)] described the preparation of particulate hydroxylapatite with stoichiometry varying from Ca/P=1.67 to 1.57. The materials so-produced contained large intercrystalline pores. It was also reported that upon heating up to 1000.degree. C. the calcium-deficient hydroxylapatites underwent partial transformation to the whitlockite phase.
McGee (U.S. Pat. No. 3,787,900, filed June 9, 1971, issued Jan. 29, 1974) disclosed a bone and tooth prosthetic material comprising a refractory compound and a calcium phosphate compound, e.g. whitlockite.
Several attempts have been made to provide a hard, strong macroform of hydroxylapatite. However, none of the previously known forms of hydroxylapatite has proven fully satisfactory. Thus, Della M. Roy and S. K. Linnehan [Nature, 247, 220 (1974); U.S. Pat. No. 3,929,971, filed Mar. 30, 1973, issued Dec. 30, 1975] described an elaborate hydrothermal exchange process whereby the skeletal calcium carbonate of marine coral was converted to hydroxylapatite. The material so produced necessarily retained the high porosity characteristic of the coral structure and moreover had a relatively low tensile strength of about 270-470 psi, a serious disadvantage in a prosthetic material.
Monroe, et al. [Journal of Dental Research 50, 860 (1971)] reported the preparation of a ceramic material by sintering compressed tablets of synthetic hydroxylapatite. The material so produced was actually a mixture of hydroxylapatite and approximately 30 percent .alpha.-whitlockite, which is Ca.sub.3 (PO.sub.4).sub.2 or tricalcium phosphate, as an ordered mosaic array of polyhedral crystallites, and appeared too porous to make it suitable for use in a dental material.
Rao and Boehm [Journal of Dental Research 53, 1351 (1974)] disclosed a polycrystalline form of hydroxylapatite prepared by isostatically pressing powdered hydroxyapatite in a mold and isothermally sintering the molded form. The resulting ceramic was porous and had a maximum compression strength of approximately 17,000 psi.
Bhaskar et al. [Oral Surgery 32, 336 (1971)] described the use of a biodegradable calcium phosphate ceramic material to fill bone defects. The material is highly porous, is resorbed from the implant site and lacks the strength of a metal or nondegradable ceramic implant.
W. Hubbard (Ph.D. Thesis, Marquette University, 1974) disclosed the sintering of compressed tablets of commercially available tribasic calcium phosphate to produce ceramic materials comprising hydroxylapatite and mixtures of the latter with whitlockite. These materials however were of relatively low density and never exceeded about 40,000 psi in compression strength.
In the field of ceramics in general much effort has been and continues to be expended in an effort to efficiently and economically produce high density ceramics. The techniques most commonly employed generally involve preparation of inorganic powders which are compacted under pressure and then sintered. Powder preparation frequently requires grinding, milling and sieving; and often the sintering must be carried out under pressure. These multistep procedures which also require high pressures are both time-consuming and expensive. Accordingly continuing effort is being made to improve and simplify the methods of ceramics fabrication as indicated by the following references which appear to constitute the most pertinent prior art.
Kamigaito et al. (U.S. Pat. No. 3,903,230, issued Sept. 2, 1975) disclosed a method for producing silicon nitride base ceramics by heating mixed powders of silicon nitride, aluminum, and aluminum nitride under a high pressure or no pressure. However, the specification states that the density of the sintered material was increased if the sintering was carried out under pressure and the single example of sintering in the absence of applied pressure resulted in a somewhat porous ceramic.
Wainer et al. (U.S. Pat. No. 3,096,144, issued July 2, 1963) disclosed a method of making inorganic oxide filaments by drying a thin film of a colloidal dispersion of an inorganic oxide to produce porous fibers of the latter which were then sintered to produce dense fibers. The process is of course limited to the production of inorganic filaments.
Cox (U.S. Pat. No. 3,278,263, issued Oct. 11, 1966) and Robbins (U.S. Pat. No. 3,778,373, issued Dec. 11, 1973) disclosed ferromagnetic chromium dioxide and iron-containing ferromagnetic chromium oxide each of which were prepared by essentially the same process which involved precipitation of the chromium oxides from aqueous solution followed by heating or calcining at 200.degree. C.-1000.degree. C. Production of the ultimate product, however, required oxidation at elevated pressure and temperature.
Grimes et al. (U.S. Pat. No. 3,826,755, issued July 30, 1974) disclosed a process for precipitating gels of various metal oxides by complexing a metal ion with a water-soluble organic polymer followed by reaction with hydroxide ion. The gels could then be dried and fired. It appears that the presence of the organic polymers in the gels would preclude formation of substantially non-porous ceramics on firing.
Miller (U.S. Pat. No. 3,066,233, issued Nov. 27, 1962) disclosed ferrite transducers which were produced by mixing milled ferrite powder with binders, gelling agents and wetting agents to give a thick gel which was then aerated, poured into a mold and fired. The product had a cellulated, sponge-like construction.
3. Patent Activities of Others
Terwilliger et al. (U.S. Pat. No. 3,992,497, issued Nov. 16, 1976) disclosed a method of sintering silicon nitride powders in the absence of pressure to effect densification thereof. However this method required that the silicon nitride and a sintering aid (MgO) be subjected to a complex wet-milling process, dried and dry-pressed in a mold at 50,000 psi prior to sintering.